1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing a toner image on a recording medium, an image forming apparatus incorporating the fixing device, and a fixing method performed by the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device is requested to shorten a first print time taken to output the recording medium bearing the toner image onto the outside of the image forming apparatus after the image forming apparatus receives a print job. Additionally, the fixing device is requested to generate a sufficient amount of heat even when a plurality of recording media is conveyed through the fixing device continuously at increased speed for high speed printing.
To address these requests, the fixing device may employ a thin endless belt having a decreased thermal capacity and therefore heated quickly by a heater. FIG. 1 illustrates such fixing device 100R1 that incorporates a thin endless belt 901. For example, as shown in FIG. 1, a pressing roller 904 is pressed against a substantially tubular, metal thermal conductor 902 disposed inside a loop formed by the endless belt 901 to form a fixing nip N between the pressing roller 904 and the endless belt 901. A heater 903 disposed inside the metal thermal conductor 902 heats the endless belt 901 via the metal thermal conductor 902. As the pressing roller 904 and the endless belt 901 rotate and convey a recording medium P bearing a toner image T through the fixing nip N, the endless belt 901 and the pressing roller 904 apply heat and pressure to the recording medium P, thus fixing the toner image T on the recording medium P. Since the heater 903 heats the endless belt 901 via the metal thermal conductor 902 that faces the entire inner circumferential surface of the endless belt 901, the endless belt 901 is heated to a predetermined fixing temperature quickly, thus meeting the above-described requests of shortening the first print time and generating heat sufficiently.
However, in order to shorten the first print time further and save more energy, the fixing device 100R1 is requested to heat the endless belt 901 more efficiently. To address this request, a configuration to heat the endless belt 901 directly, not via the metal thermal conductor 902, is proposed as shown in FIG. 2.
FIG. 2 illustrates a fixing device 100R2 in which the heater 903 heats the endless belt 901 directly. Instead of the metal thermal conductor 902 depicted in FIG. 1, a nip formation plate 905 is disposed inside the loop formed by the endless belt 901 and presses against the pressing roller 904 via the endless belt 901 to form the fixing nip N between the endless belt 901 and the pressing roller 904. Since the nip formation plate 905 does not encircle the heater 903 unlike the metal thermal conductor 902 depicted in FIG. 1, the heater 903 heats the endless belt 901 directly, thus improving heating efficiency for heating the endless belt 901 and thereby shortening the first print time further and saving more energy.
However, the fixing device 100R2 in which the heater 903 heats the endless belt 901 directly may cause cold offset due to a decreased temperature of the endless belt 901 that is too low to soften toner particles of the toner image T on the recording medium P. Accordingly, a part of the toner particles may peel off the recording medium P, resulting in fixing failure.
For example, when the fixing device 100R2 finishes a first print job performed after the fixing device 100R2 is powered on, the fixing device 100R2 may enter a sleep mode in which the heater 903 is turned off or a standby mode in which the heater 903 maintains the endless belt 901 at a standby temperature lower than a fixing temperature at which the toner image T is fixed on the recording medium P. Prior to the first print job, the fixing device 100R2 is warmed up for a substantial time so that the endless belt 901, the pressing roller 904, and the nip formation plate 905 are heated to the predetermined fixing temperature. Hence, the nip formation plate 905 stores a sufficient amount of heat during the first print job and therefore does not draw heat from the endless belt 901, preventing cold offset.
Conversely, prior to a second print job subsequent to the sleep mode or the standby mode, the fixing device 100R2 is warmed up for a shortened time because the components surrounding the endless belt 901 that are already heated during the first print job do not draw heat from the endless belt 901 and therefore the endless belt 901 is heated to the predetermined fixing temperature quickly. Accordingly, the nip formation plate 905 may not store a sufficient amount of heat within the shortened warm-up time prior to the second print job and thereby may draw heat from the endless belt 901 during the second print job, thus decreasing the temperature of the endless belt 901, which may cause cold offset.